On July 4, the Kurdish-dominated Syrian Democratic Forces (SDF), backed by the U.S.-led coalition against the Islamic State (also known as ISIS), broke into the Old City of Raqqa, the jihadist pseudo-state’s de facto Syrian capital. Although fighting in Raqqa will continue to be a bloody grind, when it is over, the battle will be the capstone of what is now a successful, years-long collaboration between the U.S.-led coalition and the SDF.

But the battle against ISIS will not end with the liberation of Raqqa, and neither will the United States’ commitments in Syria. Washington’s approach to fighting ISIS in Syria—and in particular the local Kurdish partner it has chosen—has granted a victory that can last only as long as the United States stays. If one looks beyond Raqqa and the immediate campaign against ISIS, the bigger strategic picture is alarming: the United States has set itself up for an indefinite presence in northeast Syria, in the middle of an unsettled and unfriendly region, with no obvious way to leave.

DIFFICULT ALLIES

The SDF is a Syrian force led by the Kurdish People’s Protection Units (YPG), with which the coalition struck up a tactical partnership against ISIS in 2014. In 2015, the U.S. military helped rebrand the YPG and its smaller, subordinate allies as the “SDF,” apparently in order to put the group in a slightly less controversial package. The YPG is closely linked to the Kurdistan Workers’ Party (PKK), which has waged a decades-long insurgency in neighboring Turkey, Washington’s NATO ally. Further, the YPG and its civilian parallel, the Democratic Union Party (PYD), are seeded throughout with PKK-trained cadres. This has not been lost on Turkey, which sees the PYD-YPG as the PKK’s Syrian affiliate. Since 2014, Ankara has watched the group’s territorial and numerical expansion with alarm, even as it has grappled with a revived PKK insurgency inside Turkey. The United States has set itself up for an indefinite presence